The invention relates to the production of circuits by microlithography and more specifically to a process for the optical alignment of masks and substrates located in close-up planes i.e., planes in close proximity.
The production of integrated circuits involves the formation on a substrate of windows making it possible to position the installation or treatment. This substrate is covered with a photosensitive resin layer. The windows are produced by masking this resin from a mask. Transfer has been carried out by direct contact or proximity and more recently by the optical projection transfer method. Although the proximity transfer method has been in existence for a considerable time, it is still valid when the radiant energy source is e.g. constituted by a source emitting in the X-ray range.
Thus, in means for reproducing masks by a cast shadow using a high energy radiation source, such as X-rays, electrons and ions, it is possible to reproduce patterns or motifs having dimensions much smaller than a micrometer and down to 1/10 micrometer, whilst retaining a separation distance between the planes of the masks and the substrate of several micrometers, in order to prevent any contact, which constitutes a source of damage. Under these conditions, the effects of photon or corpuscular diffraction are negligible.
As a result of the absence of image-forming optical elements, the resolving power of these systems is independent of the dimensions of the useful field covered, which makes it possible to envisage the production of micronic and submicronic circuits by projecting masks having a large surface area, diameter of 100 mm or more, so that it is possible to achieve very high production rates for such circuits.
An apparatus for the reproduction of patterns using this procedure has a substrate which is exposed to the action of X-rays or more generally to radiant energy radiation through an appropriate mask, located at a few micrometers from the substrate. Each operation requires a precise alignment of the mask and the substrate. In order to obtain this alignment with an accuracy of about 1/10 micrometer, several alignment processes using the properties of diffracting light by gratings have been described, e.g. the Moire fringes method, or the Torii and Smith methods. These methods consist of inscribing on the mask and the substrate, gratings having predetermined spacings and then detecting the radiation diffracted by these two gratings located in the close-up planes.
However, all these methods leave an uncertainty as a result of the periodicity of the detected signal and it is then necessary to remove this uncertainty by another method.
To obviate these disadvantages, it has been proposed in U.S. Pat. No. 4,311,389, to align two close-up planes and the alignment apparatus utilizing this process makes it possible to obviate the disadvantages of the aforementioned processes, whilst in particular making it possible to obtain a very accurate alignment of the two close-up planes and to tolerate, without alignment precision loss, larger spacing variations of the two planes than in the prior art alignment processes.
To this end, a lens with linear Fresnel zones is inscribed on the mask and a line, whose width is the same or greater than the width of the smallest Fresnel zone inscribed on the mask is inscribed on the substrate. The illumination of the mask by a parallel monochromatic light beam, e.g. a laser, makes it possible to form a rectangular light spot (focus) on the substrate and this corresponds to the order of diffraction +1 which, on covering the marking line formed on the substrate, leads to the detection of an illumination maximum or minimum of the radiation reflected by the substrate depending on whether the mark inscribed on the substrate has a larger or smaller reflecting action than the zone surrounding it.
This process makes it possible to align under good conditions, all the points of maximum diameter zones, typically equal to 10 mm, for line widths down to 0.2 micrometers. More generally, the maximum permissible zone diameter corresponds to approximately 10.sup.5 resolution points, which does not make it possible to take advantage of all the possibilities offered by the cast shadow reproduction process and which were referred to hereinbefore.
This limitation is essentially due to the relative dimensional distortions which inevitably occur on increasing the dimensions of the mask and correlatively the substrate, because transfer takes place with a 1/1 magnification.
Thus, other than by dividing the mask and the substrate into elementary zones to be individually aligned, it is not possible to use a mask with a diameter of 100 mm or more in a continuous manner for the aforementioned line width.
However, it would be possible to use larger masks, if it was possible to place means for compensating the relative dimensional distortions of the mask and the semiconductor wafer after each stage of the circuit production process, in order to guarantee the precise alignment of all the patterns on the complete field to be covered.